You could also do:
index.html:
Code
<html><head>
<meta http-equiv="refresh" content="0;URL=forums/ubbthreads.php">
</head>
<body>
</body>
</html>

Or index.php
Code
<?php
header("Location: http://www.yoursite.tld/forums/ubbthreads.php");
exit;
?>

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